a) Field of the Invention
The present invention relates to an objective lens system which uses rays within the near ultraviolet region around 250 nm.
b) Description of the Prior Art
The objective lens system which use rays in the near ultraviolet region around 250 nm are applied to high resolution imaging optical systems, research of living organisms by utilizing the ultraviolet rays as well as research and application of laser lights and so on. Though the reflection type of objective lens systems is used widely in these fields of application, the refraction type of objective lens systems are more advantageous for research of living organisms by utilizing the ultraviolet rays and laser lights as well as the other application for which high resolution is required.
A refraction type of objective lens system which uses the ultraviolet rays is described on pages 689-692, Vol. 38 of the J.Opt. Soc.Am. This objective lens system is of a glycerine oil immersion type, and has a high magnification, a large numerical aperture and favorably corrected spherical aberration.
Further, the objective lens systems for the ultraviolet region disclosed by Japanese Patent Kokai Publication No. Hei 1-319719 and No. Hei 1-319720 have low magnifications and small numerical apertures, but spherical aberration corrected relatively favorably with small numbers of lens elements.
However, the objective lens system described in the J.Opt. Soc.Am. has remarkable curvature of field and a very narrow effective visual field, and is of the glycerine oil immersion type which cannot be applied to the research of semiconductors and laser lights.
Furthermore, the objective lens system disclosed by each of the above-mentioned Japanese patents has low magnification and small numerical aperture owing to a fact that it is composed of a small number of lens elements, and is nevertheless insufficient in correction of chromatic aberration and curvature of field.